This invention relates to a case for a wristwatch.
Wristwatches are generally composed of a movement for measuring and displaying time and a case for protecting the movement as is widely known. There is now a trend towards a conversion from mechanical to electronic movements for the sake of obtaining higher accuracy, and watch cases, in addition to protecting the movement, must be designed and constructed from the viewpoint of ornamentation as well as function where modern wristwatches are concerned. Research and development in this field is proceeding.
Wristwatch cases are frequently subjected to harsh conditions and severe use. It is therefore necessary to fabricate these cases from materials which exhibit weather and scratch resistance while at the same time maintaining an attractive appearance, such materials being limited to brass, stainless steel, synthetic resins, hard alloys, and the like. Each of these materials possess both advantages and disadvantages; brass, for example, is easily machined but exhibits poor corrosion resistance if its surface is not plated; hard alloys are attractive and remarkably weather resistant but are difficult to machine; and synthetic resins, although they are readily machined and weather resistant, are much less attractive than the other materials mentioned. Stainless steel is comparatively easy to machine, exhibits excellent weather resistance and possesses a characteristic appearance of high-quality, accounting for its widespread use as a material for constructing modern watch cases. However, when fabricating a watchcase even stainless steel requires a high level of machining technique and numerous processing steps in comparison to brass or synthetic resin, a fact which inevitably leads to high production costs. Stainless steel is therefore unsuitable for fabricating inexpensive wristwatch cases.
In view of these circumstances, a number of proposals have been made which contemplate the production of a watch case by subjecting a thin metal plate such as stainless steel or brass to a press operation in order to form a shell the inside of which is provided with a synthetic resin that is unified with the shell. Such a case maintains the external appearance of the stainless steel but is much easier to machine. Nevertheless, these cases involved a large number of defects.
First, there is the case where the shell fabricated from the thin metal sheet and the synthetic resin portion to be provided inside the shell are formed individually and then integrated. The synthetic resin portion when formed undergoes contraction and is thus integrated with the already formed shell by bending one end of the shell. Even so, there is still a gap between both members which invites the penetration of water, perspiration and other contaminants which can foul the wristwatch during use. To overcome this problem it is necessary to grind the synthetic resin portion so as to obtain the prescribed dimensions. This has little merit in so far as lowering costs is concerned and has not been put to practical use.
Second, there is a case where a casting synthetic resin is poured inside the shell so as to form the watch case. In this instance an extended period of time is required for the liquid resin to harden after it has been poured. It is also difficult to maintain constant polymerization conditions so that it becomes impossible to uniformly maintain the cast resin composition. Moreover, water-proofing ability becomes a problem since air bubbles remain in the resin after hardening.
Third, a synthetic resin for injection molding can be poured inside the shell to form the watch case. In general, the watch case possesses a ring-shaped configuration and a varying, complex cross-section which includes a continuous portion for a band and holes for external control members. As a result, the synthetic resin portion begins to exhibit a directional property at the time of injection molding regardless of the injection conditions, and the deformation which accompanies this phenomenon appears at the inner and outer diameter of the synthetic resin portion. Accordingly, defects arise which are identical to those that occured in the first example.
Although a watch case obtained by equipping the inside of a shell fabricated from a thin metal plate with a synthetic resin member appeared to be promising in theory, the watch cases of this type actually possessed numerous problems as outlined above and were not highly appraised in terms of low cost and appearance.
As the result of intensive research and experimentation with regard to watch cases in which a synthetic resin portion is disposed inside a shell, it has been discovered that the fatal flaws of the prior art, namely the gap between the shell and the resin portion as well as the deformation of the resin portion along its inner diameter, can be eliminated if an annular reinforcing member is disposed inside the shell along with an injection molding resin provided in such a manner as will allow it to retain the reinforcing member. It has also been discovered that an extremely inexpensive watch case can be obtained by combining watch case constituent elements such as packing or a threaded portion with the annular reinforcing member or the synthetic resin member.